Combining mesh from skin modifier to manifold object
Instead of decimate, try the re-mesh modifier.  The re-mesh modifier should be able to handle all of the hair strands and their intersections.  It should act as a global "boolean union" like you want.

If this doesn't work, you could remove the boolean modifiers, export it to a STL file, and then run it through https://modelrepair.azurewebsites.net/ I've had a LOT of luck with this tool.